Segmented D/A converter with enhanced dynamic range

ABSTRACT

A digital-to-analog converter having series-connected transistors forming high impedance current sources for respective segmented resistor strings. A series transistor forming a current sink for one resistor string presents a high output impedance by utilizing a negative feedback amplifier. The effects of a headroom resistor and an offset resistor in one resistor string are negated by configuring an output amplifier with appropriate gain resistors. A highly accurate D/A conversion can be achieved by utilizing all resistors of the main and sub-resistor strings with the same value.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.09/583,341 filed on May. 31, 2000 now U.S. Pat. No. 6,384,763 entitled“SEGMENTED D/A CONVERTER WITH ENHANCED DYNAMIC RANGE” and is related toU.S. patent application Ser. No. 09/584,217 entitled “D/A ResistorStrings with Cross Coupling Switches” and U.S. patent application Ser.No. 09/584,311 entitled “D/A Converter Street Effect Compensation”.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to digital-to-analogconverters, and more particularly to the segmented type of converterhaving multiple resistor sections for carrying out the conversionprocess.

BACKGROUND OF THE INVENTION

In mixed signal circuits which involve both analog and digital signals,circuits are generally required for converting the analog signals intocorresponding digital signals, and vice versa. Digital-to-analogconverters provide an analog output as a function of the digital input.Many different varieties of conversion circuits are commerciallyavailable to satisfy the various constraints required, such a speed,resolution, differential non-linearity, monotinicity, etc. Thedigital-to-analog conversion process can be carried out according todifferent techniques, including the use of weighted current sources, R-2ladder networks, as well as many other well-known conversion schemes.Because of the wide utilization of microprocessors employed to processdigital information, it is a common practice to integratedigital-to-analog converters on the microprocessor chip. Because the useof chip area is always a concern, the minimization of components istherefore of paramount importance.

In a rudimentary digital-to-analog converter having a 12-bit resolution,as many as 4,096 series resistors can be utilized to produce a differentmagnitude of analog voltage in response to each of the 4,096 digitalwords. The amount of semiconductor space required for all theseresistors would be prohibitively large. In addition to each resistor,there are required corresponding switches across each resistor forselecting voltage levels in response to different input digitalcombinations.

A segmented digital-to-analog converter (DAC) provides an adequatesolution to the problem of a large number of resistors to carry out theconversion algorithm. FIG. 1 of the drawings illustrates two resistorsegments or strings of a 12-bit DAC 10. The digital-to-analog converter10 includes a main DAC 12 and a subsidiary (“sub”) DAC 18. The main DAC12 includes a number of series resistors to provide 2^(x) differentanalog levels in response to X most significant digital bits.

A sub-DAC resistor string 18 includes a number of series-connectedresistors to provide 2^(y) different analog levels for the leastsignificant bits of the DAC 10. The DAC 10 includes X+Y digital inputbits, and produces 2^((x+y)) analog levels. A fewer number of resistorsare required in a segmented DAC driven by a corresponding number ofdigital input bits.

A first switch multiplexer 20 is connected between the main DAC resistorstring 12 and the sub-DAC resistor string 18. The switch multiplexer 20is of conventional design for allowing the sub-DAC resistor string 18 tobe connected in parallel to any one or more of the resistors in the mainDAC resistor string 12. The switch multiplexer 20 is required to provideconnections to 2^(x) different resistor combinations in the main DAC 12.

The sub-DAC 18 also includes a switch multiplexer 22 for selecting 2^(y)different resistance values. The output 24 of the second switchmultiplexer 22 is connected to an operational amplifier 26. An output 28of the amplifier 26 provides 2^((x+y)) different analog outputscorresponding to the different combinations of the X+Y digital bitsapplied to the DAC 10.

While the switch connections between the main DAC resistor string 12 andthe sub-DAC resistor string 18 provides a multiplying function andreduce the number of resistors required to complete the X+Y bitconversion, various shortcomings of this arrangement exist. For example,the coupling of the sub-DAC resistor string 18 to the main DAC resistorstring 12 can present an unbalanced load thereon, as a function of theoverall resistance of the sub-DAC resistor string 18. This can occurwhen the individual resistors of the string 18 are switched in or out ofthe circuit. When an unbalanced load is connected across the main DACresistor string, a nonlinear conversion results. While FIG. 1illustrates that only the main DAC resistor string 12 is supplied with areference supply voltage, the same loading factor exists when thesub-DAC resistor string 18 is independently powered by a referencesupply voltage. Various attempts have been made in the prior art toovercome this loading problem between the main and sub-DAC resistorstrings. As noted in the background portion of U.S. Pat. No. 4,338,591by Tuthill, there is proposed the remedy of placing a buffer amplifierbetween the main DAC resistor string and the sub-DAC resistor string.The buffer amplifiers do effectively isolate the main and sub-DACresistor strings. But, substantial semiconductor area is required toisolate the resistor strings with a pair of buffer amplifiers. Also, thedynamic range of the main DAC is severely limited by the input range andthe finite common mode rejection of the buffer amplifiers.

Instead of isolating the main DAC and the sub-DAC resistor strings withbuffer amplifiers, the use of current sources is suggested in U.S. Pat.No. 5,703,588 by Rivoir et al. By utilizing a constant current to drivethe main DAC and another constant current source to drive the sub-DAC, abalanced operation therebetween can be accomplished so that neitherresistor string draws current from the other. The loading problem isthus alleviated, irrespective of the switch connections. When utilizingcurrent sources, it is imperative that the output impedance thereof issome orders of magnitude higher than the voltage of the resistor stringbeing driven. Otherwise, inaccuracies in the conversion process becomesignificant, especially when large voltage excursions in the sub-DAC areexperienced.

Current mirrors are well known for use as current sources and currentsinks in DAC resistor strings. While accurate current control can beachieved, the output impedance of such a structure is not always as highas desired. Utilizing two transistors in series as either a currentsource or a current sink in a DAC resistor string could increase theoutput impedance of the current source by a factor of the gain of thesecond transistor. This solution can cause other problems.

It can be seen from the DAC 10 shown in FIG. 1 that, depending on theswitch setting of the switch multiplexers 20 and 22, analog voltagesvery near the reference voltage, or very near the circuit common voltage(ground) can be coupled to the output amplifier 26. Unless expensive,precision instrumentation amplifiers are utilized, a wide dynamic inputrange (rail-to-rail) of amplifiers is not always available. However, itis most desirable to design DAC resistor strings that operate“rail-to-rail”, otherwise wasted voltage ranges due to headroomresistors must be used. In other words, to reduce the dynamic range overwhich the amplifier must operate, resistors can simply be placed inseries at the top and/or bottom of the DAC resistor strings. Suchresistors waste power and require additional semiconductor area.

The accuracy in the conversion of the digital input to an analog outputis a function of the values of the resistors with which the resistorstrings are formed in the semiconductor material. While exact valueresistors are difficult to form in integrated circuits, therepeatability of making a nominal resistance value is high.

The value of the DAC resistors themselves is only one consideration informing or scaling accurate resistance values for the resistor strings.Another consideration is the contact resistance for each semiconductorresistor. Since each resistor has two contacts, there may be a contactresistance in series with each semiconductor resistor. U.S. Pat. No.5,343,199 illustrates DAC resistor strings where the various segments ofa resistor string are connected together by metal contacts. Whenutilizing resistors of different values, the contact resistance presentsa problem when attempting to scale the resistor values. For example, ifthe resistance for each contact is 10 ohm. Then the total resistance fora nominal 1 k resistor would be 1,020 ohm. If it were desired to makeone resistor that is exactly ten times the 1k value, then the totalresistance of that resistor would be 10,020 ohm. The scaling ofsemiconductor resistors is thus difficult. In various DAC resistorstrings, the values of resistors can be multiples of the others, andthus it is difficult to make the resistors with any degree of precision.

Even when the contacts are not directly in series with the individualresistor elements of the resistor string, the process variations informing the contacts to the various resistor elements can adverselyaffect the values of the resistors. In those situations when theresistor string itself is one continuous resistor formed as apolysilicon strip or some other resistive material, or otherwise, theformation of a contact or tap overlying and in electrical contact withthe polysilicon strip may affect the resistance of that part of thepolysilicon underlying the contact.

It has been found that the repeatability of resistors having desiredvalues is affected by the location of the resistors on the semiconductormaterial, as a function of what electrical components are formedadjacent to the resistors. Resistors formed adjacent other resistorstend to be more uniform in value than resistors formed near differentcomponents, such as transistors, capacitors, etc. Hence, it has beenfound that by forming dummy resistors on each lateral side of a numberof active resistors, the active resistors are better matched in value.While the formation of dummy resistors adjacent the active resistorsimproves the resistor value matching, additional semiconductor area isrequired for nonactive resistor components.

The variation in resistor values as a result of semiconductor processingis realized by those skilled in the art, and as set forth in U.S. Pat.No. 5,343,199 by Sugawa. Each resistor string of the D/A converter isfabricated as two separate resistor strings, but with various nodes ofone resistor string short circuited to the same voltage nodes of theother resistor string. Moreover, the voltage strings are cross-connectedin a reverse manner so that the supply voltage of one end of oneresistor string is the ground end of the companion resistor string. Inthis manner, with variations in resistance values due to processdifferences, the nodes are forced to the same voltage by the metal crossconnects. Although additional metal cross connects are required, as wellas the semiconductor area required to accommodate the metalization, thenon-linear conversion error is reduced. However, when a voltage isextracted at a node of the resistor string that is not short circuitedto a similar-voltage node of the companion resistor string, there canstill exist a non-linear error due to the processing difference ofresistors values.

It can be seen that a need exists for a DAC that utilizes currentsources for isolating the DAC resistor strings, but where such currentsources have a high output impedance, without substantially affectingthe output rail-to-rail operation of the DAC. Another need exists for adigital-to-analog converter having a main DAC resister string and asub-resistor string, and where the current which drives the main DACresistor string is replicated in a specified ratio to the sub-resistorstring. Another need exists for a DAC that utilizes resistors formedwith a higher degree of repeatability to thereby enhance the accuracy ofthe conversion process.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein, in one aspectthereof, comprises a segmented DAC including a main DAC resistor stringdriven by a highly accurate reference voltage buffer which drives themain DAC resistor string with a specified current. The current in thesub-DAC resistor string is a replica of the current in the main DACresistor string. In the preferred form of the invention, the currentsource driving the sub-resistor string mirrors the current in the mainDAC resistor string. The sub-resistor string current source operatesfrom the top voltage rail and is formed as two transistors operating inseries, thereby substantially increasing the output impedance thereof.The use of a headroom resistor in the main DAC resistor string allowsthe cascode current source of the sub-DAC resistor string a sufficientoperational voltage range.

The bottom end of the main DAC resistor string is terminated in anoffset resistance, thereby providing an intentional offset voltage. Aregulated current sink is connected to the bottom end of the sub-DACresistor string. The offset voltage allows a sufficient operationalvoltage range for the current sink. Although the current sink has only asingle transistor in series with the sub-DAC resistor string, it isconfigured with negative feedback to increase the output impedancethereof.

Because the main DAC resistor string does not operate rail-to-rail, theoutput amplifier of the DAC is configured to produce a gain such thatthe output voltage of the DAC swings from substantially the referencevoltage to ground. The headroom voltage and the offset voltageintroduced in the D/A conversion circuits are thus negated in the outputamplifier stage.

In order to make the DAC with accurate resistance values, the resistorsof each string have a nominal resistance value. In this manner, thecontact resistance is scaled together with the overall resistance ofeach resistor.

According to another aspect of the invention, the headroom resistorutilized in the main DAC resistor string also functions as a dummyresistor for compensating the street effect of the main DAC resistorstring. Preferably, the headroom resistor is formed in the semiconductormaterial as N equal valve resistors, in close proximity to the Nresistors of the main DAC resistor string.

According to a further aspect of the invention, the non-linearconversion error is reduced by forming the main DAC resistor string astwo sets of resistor strings. Transistor switches are employed to crossconnect similar voltage nodes of each set of main DAC resistor strings.Should the nodes be at slightly different voltages due to differentresistor values resulting from semiconductor process variations, theswitches will average the node voltages. Moreover, the switches areconfigured to select the voltage across one resistor of each set of MainDAC resistor string sets, and couple such voltage to the sub-DACresistor string.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying Drawings in which:

FIG. 1 illustrates a rudimentary digital-to-analog converter constructedaccording to prior art techniques;

FIG. 2 illustrates the details of a DAC constructed according to theprinciples and concepts of the preferred embodiment of the invention;

FIG. 3 illustrates the relative physical layout of a headroom resistorthat also functions as a dummy resistor to the main DAC resistor string;and

FIG. 4 illustrates a physical layout of the main DAC resistor string,and its companion resistor string, with the switch multiplexerimplemented as corresponding transmission gates.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates the details of the segmented DAC constructedaccording to a preferred embodiment. The DAC 30 incorporates a number offeatures, and overcomes the problems and disadvantages attendant withthe prior art converters. The DAC 30 is shown with a main resistorstring 32 comprised of two hundred fifty-six resistors, and asub-resistor string 34 comprised of sixteen resistors. The main resistorstring 32 can produce two hundred fifty-six different analog voltagelevels, corresponding to the different combinations of eight mostsignificant digital input bits. The sub-resistor string 34 can producesixteen different analog voltage levels, corresponding to four leastsignificant digital input bits. A 12-bit D/A conversion is thusachieved. According to an important aspect of the invention, eachresistor of the main string 32 is fabricated with a nominal resistancevalue. Each resistor of the sub-resistor string 34 is also formed withthe same nominal value resistance. In the preferred form of theinvention, the nominal resistance value of each resistor is about sevenhundred ohm.

A first switch multiplexer 20 is adapted for allowing the voltage acrosseach resistor of the main resistor string 32 to be coupled across thesub-resistor string 34. The multiplexer switch 38 as shown is effectiveto couple the voltage at the top node of resistor 40 of the mainresistor string 32, to the top node of the resistor 42 of thesub-resistor string 34. Switch 44 of the multiplexer 20 is effective tocouple the voltage at the bottom node of main resistor string resistor40, to the bottom node of the resistor 46 of the sub-resistor string 34.The switch multiplexer 20 is essentially identical to that well known inthe art, as shown in FIG. 1 above, or in FIG. 3 of U.S. Pat. No.5,703,588. As noted above, the plural switches of the multiplexer 20 areconfigured to provide switch connections to the various resistors toprovide two hundred fifty-six different analog voltage levels across theconductors coupling the main resistor string 32 to the sub-resistorstring 34.

A second switch multiplexer 22 is adapted for accessing voltages at thedifferent resistor nodes of the sub-resistor string 34. Sixteendifferent switches of the second multiplexer 22 are adapted foraccessing the analog voltage at sixteen different resistor nodes of thesub-resistor string 34. The voltage is coupled via only one closedswitch of the multiplexor 22 to a non-inverting input of an outputamplifier 50. With two hundred fifty-six different analog voltagesprovided by the main resistor string 32, and sixteen different analogvoltages produced by the sub-resistor string 34, four thousandninety-six (4,096) different analog voltages can be provided to theoutput amplifier 50. This corresponds to twelve bits of digitalresolution. Those skilled in the art can readily understand thecircuitry involved to convert the input digital signals to correspondingswitch operations of both the main and sub-switch multiplexers 20 and22.

As noted above, each resistor of the main resistor string 32, as well asthe sub-resistor string 34, are constructed with a substantiallyidentical resistance. More accurate analog voltage levels are thusachieved. Although not shown, each DAC resistor includes two contacts,but such contacts are not formed either in series with the resistorsegments, or thereover. Thus, each resistor can be made withsubstantially the same value. As such, a much better match between theoverall resistance values of the main and sub-resistor strings isachieved.

The main resistor string 32 is driven by a precision reference voltage.Based on the composite resistance of the main DAC resistor string, acurrent of specified magnitude flows therethrough. The sub-resistorstring 34 is driven by a current source. As noted above, a highimpedance current source is desirable in order to further isolate anychanges in supply voltage and render the same independent of the outputvoltage obtained from the resistor strings. The output impedance ofcurrent source transistor 72 is increased by a factor defined by thegain of cascode-connected transistor 74.

The main resistor string 32 draws current through a pair ofseries-connected transistors 60 and 62. These transistors mirror thecurrent in the main resistor string 32 in a desired ratio to thesub-resistor string 34. The manner in which the respective transistors60 and 72, as well as transistors 62 and 74 are interconnected, assuresthat the current that drives the main resistor string 32 is replicatedin the manner noted to the sub-resistor string 34. A current I is driventhrough a headroom resistor 54, through the main resistor string 32, andfinally through an offset resistor 56. The headroom resistor 54 is notpart of the main resistor string 32 in terms of producing an analogvoltage level for D/A conversion, but functions to provide a headroomvoltage for the current source at the top of the resistor string 34. Inthe preferred form of the invention, the headroom resistor 54 is of thesame value as the composite resistance of the main resistor string 32.The offset resistor 56 is preferably a smaller value than the headroomresistor 54. In one aspect of the invention, the offset resistor 56 isone-eighth (⅛) the value of the headroom resistor 54. The function ofthe offset resistor will be described in more detail below.

As noted above, series-connected transistors 60 and 62 form a cascodetransistor circuit for supplying a current of the value I to the mainresistor string 32. The transistors 60 and 62 also form a circuit formirroring the current therethrough to the sub-resistor string 34. A biasvoltage 64 supplies a voltage to the gate of transistor 62. Anoperational amplifier 66 drives the gate of transistor 60 with avoltage. The non-inverting input of the operational amplifier 66 isconnected to a reference voltage 68. The inverting input of theamplifier 66 is connected to the junction 70 between the headroomresistor 54 and the source terminal of current source transistor 62.With this arrangement, the junction 70 is maintained at the referencevoltage Vref. The precision voltage Vref is maintained at the junction70 of the main resistor string 32. The current I which drives the mainDAC resistor string 32 is thus a function of the magnitude of thereference voltage at node 70, plus the composite resistance of theheadroom resistor 54, the resistance of the main DAC resistance string,and the resistance of the offset resistor 56. As can be appreciated,this current may change because of temperature considerations, theswitched resistor connections to the sub-resistor string, referencevoltage variations, changes in parameters of the main resistor string,etc. What is important is that because of the mirroring of the currentfrom the main DAC resistor string 32 to the sub-resistor string 34, andchange in the former is reflected in the latter. The maintenance of thiscurrent relationship reduces any loading affect that the sub-resistorstring 34 would otherwise have on the main DAC resistor string 32.

The sub-resistor string 34 is driven by a mirroring current source. Thesub-resistor string current source constitutes a series-connectedtransistor arrangement to thereby increase the output impedance thereof.Series-connected transistors 72 and 74 provide a very high outputimpedance to the sub-resistor string 34. The headroom resistor 54 in themain DAC resistor string 32 allows the current source transistors 72 and74 to operate in an acceptable voltage range to supply a mirroredcurrent to the sub-resistor string 34. Although the dynamic range ofoperation of the main resistor string 32 and the sub-resistor string 34are limited because of the headroom resistor 54, such limited dynamicrange is overcome in a manner set forth below.

Since the headroom resistor 54 is the same value as the compositeresistance of the main resistor string 32, the headroom resistor 54serves an additional important role in maintaining the matching accuracyof the main DAC resistor string 32. Because of the variations insemiconductor manufacturing process, dummy resistors are often placedaround the actual resistor to compensate for the street effect in orderto maintain electrically good resistor matching properties. However,dummy resistors are otherwise nonfunctional and require additionalsilicon area and thus increase the manufacturing cost. As will bedescribed more fully below, the resistor 54 functions as both theheadroom resistor and the street effect compensation resistors.Therefore, no silicon area is unnecessarily wasted on dummy resistorsalone.

Forming a part of the mirrored current source associated with thesub-resistor string 34 is an additional pair of series-connectedtransistors, namely transistor 76 and transistor 78. Much like currentsupply transistor 60 which drives the main resistor string 32, thetransistors 72 and 76 associated with the sub-resistor string 34 aredriven by the output of the operational amplifier 66. In like manner,transistors 74 and 78 associated with the sub-resistor string 34 aredriven by the bias voltage 64. The conduction channel width of currentsource transistors 72 and 74 are constructed to provide a mirroredcurrent of I/16, as compared to main resistor string current supplytransistors 60 and 62 which supply a current I. The reason for thiscurrent difference by a factor of sixteen is that the compositeresistance value of the sub-resistor string 34 is sixteen times theresistance value of a selected main string resistor, such as resistor40. By maintaining this current ratio between the main and sub-resistorstrings, the current flow therebetween is reduced, as is the loadingtherebetween. As noted above, each resistor of the main resistor string32 is of the same value, and can be individually selected by respectiveswitches of the switch multiplexer 20. Current source transistor 76 isstructured to provide a current of I/8, one half of which is carried onconductor 80, and one half of which is carried through transistor 78.Transistor 78 and conductor 82 thus each carry a current of I/16.

A regulated current sink 84 functions to provide a high output impedanceto the current sink, while using only a single current sink transistor86 in series with the sub-resistor string 34. The current carriedthrough series transistor 86 is regulated as a function of an amplifierdefined by transistors 76 and 78, as well as current sources 88 and 94,and transistors 90, 92 and 96. The output of the amplifier is connectedto the gate of transistors 92 or 86. The current flowing throughtransistor 92 and the output of the amplifier define a negative feedbackloop which forces the current carried by transistor 92 to be equal tothe difference between the current source 88 and the current oftransistor 76. The current that flows through transistor 92 is mirroredin transistor 86 by a ratio of 1:2. Hence, a current of magnitude I/16flows through transistor 92, thereby causing a current of magnitude I/8to flow through current sink transistor 86. The amplifier and feedbackarrangement associated with current sink transistor 86 substantiallyincrease the output impedance thereof, without requiring a substantialvoltage swing for operation thereof.

The voltage developed across the offset resistor 56 connected at thebottom of the main resistor string 32 provides the operating voltagerange for the current sink transistor 86. As noted above, the currentsink transistor 86 is regulated to maintain a constant current of I/8therethrough. As also noted above, transistors 92 and 86 form a currentmirror such that series current sink transistor 86 maintains twice thecurrent therethrough, as compared to transistor 92. The current source88 maintains a bias on the gate of transistor 92 such that the currentI/16 is maintained through transistor 92.

In the amplifier feedback circuit, the current source 94 creates aconstant current of I/16 that is carried through transistor 96. Thecurrent I/16 carried by transistor 96 as well as the current I/16carried through the sub-resistor string 34, are combined and carriedthrough the current sink transistor 86 as current I/8. The transistors96 and 90 force the voltage on node 85 equal to the voltage on the drainof transistor 92. Thus, current sink transistor 86 has the exact replicabias condition as transistor 92. Once the current through the transistor92 is established via the negative feedback, the current is mirrored tothe current sink transistor 86. As noted above, the conduction channelof transistor 86 is twice that of transistor 92, whereby transistor 86carries twice the current sources, i.e., I/8. The current sources 94 and88, the transistors 90, 96 and 76 not only maintain a constant currentthrough current sink transistor 86, but also multiply the outputimpedance thereto. The output impedance of current sink transistor 86 ismultiplied by a factor of about 100, due to the negative feedbackamplifier arrangement. By utilizing only a single current sinktransistor 86, in conjunction with the feedback amplifier, theoperational voltage thereacross is much smaller than thecascode-connected current sources connected at the top portion of themain resistor string 34. With this arrangement, the current sinktransistor 86 can operate with as little as 0.1 volt of drain-sourcevoltage. As noted above, the voltage across the offset resistor 56determines the range of operating voltage across the current sinktransistor 86.

From the foregoing, the main resistor string 32 and the sub-resistorstring 34 of the D/A converter are driven by mirrored currents tomaintain an isolation between the resistor strings so that improvedconversion accuracy is achieved. In addition, a high impedance outputcurrent source and sink are provided in the sub-resistor string 34 toprovide a high degree of isolation at the respective top and bottomthereof, as well as to improve the current regulation therethrough.

As noted above, the resistor 54 located in the main resistor string 32provides operational headroom for the cascode-connected current sources,as well as device matching properties. Because the value of the headroomresistor 54 is the same as the composite resistance of the main resistorstring 32, the dynamic range of the analog voltages generated in theconversion circuit is about half the reference supply voltage. Althoughthe offset resistor 56 further reduces the dynamic range, its value isone-sixteenth ({fraction (1/16)}) that of the combined resistance of theheadroom resistor 54 and the main resistor string 32. In practice, thedynamic range of the analog voltage generated in the main DAC is about0.4706 times the reference voltage Vref. A wide operating voltage rangeis reestablished in the configuration of the output amplifier.

The output analog voltage resulting from the digital conversion iscarried by way of the second switch multiplexer 22 to the non-invertinginput of the output amplifier 50. The feedback resistor 100 and theinput resistor 102 of the amplifier 50 are of the same resistance value.The value of resistor 110 is eight times the resistance value of eitherresistor 102 or 100. By configuring the amplifier 50 as thenon-inverting amplifier, the gain thereof then becomes 2.125. Hence,because the dynamic range of the analog voltages generated by theresistor strings 32 and 34 is about 0.4706 times the reference supplyvoltage, the amplifier 50 increases the amplitude of the analog signalsby a factor of 2.125, thereby reestablishing the full dynamic range ofthe converted analog voltage. An output voltage swing of the amplifier50 from rail to rail is thus reestablished.

Configured much like operational amplifier 66, the amplifier 104 drivestransistor 106 so that a reference voltage of Vref is maintained at node108. The reference voltage biases the resistor 110 connected to theinverting input of the output amplifier 50. As noted above, the value ofresistor 110 is eight times that of either resistor 100 or 102. Thevalue of resistor 110 with regard to resistor 100 shifts the operatingpoint of the output amplifier 50, thereby overcoming the effect of theoffset voltage generated by offset resistor 56.

From the foregoing, a high precision integrated digital-to-analogconverter has been disclosed. The resistor elements of both the mainresistor string and the sub-resistor string are of identical nominalvalues, thereby improving the accuracy of the analog voltage levelsgenerated by the resistor strings. The main DAC resistor string isdriven by a precision reference buffer circuit where a reference currentis generated. By utilizing high impedance current sources andreplicating the current from the main DAC resistor string, a highlyaccurate sub-DAC output is achieved. From a matching perspective, theheadroom resistor improves the accuracy of the main DAC resistor stringby also functioning effectively as street compensation components.

FIG. 3 illustrates the physical layout of the various resistors of themain DAC resistor string 32. In practice, the resistor string 32 of FIG.2 is fabricated as two distinct resistor strings, one identified asreference numeral 120 in FIG. 3 and the other identified as referencenumeral 122. The headroom resistor 54 of FIG. 2 is also fabricated astwo distinct resistors, one identified as reference numeral 124 and theother identified as reference numeral 126. It can be seen that theheadroom resistor 124 is in a series with the main DAC resistor string120. In like manner, the headroom resistor 126 is in series with thecompanion main DAC resistor string 122. Each of the main DAC resistorstrings 120 and 122 are fabricated with 256 individual resistors ofidentical value. Each headroom resistor 124 and 126 is similarlyfabricated with 256 identical-valve resistors each. The top end of eachheadroom resistor 124 and 126 is connected to node 70 of FIG. 2. Node 70is driven by the reference buffer 52, and a current supply sourcedefined by transistors 60 and 62 to provide low impedance output to themain DAC resistor strings 120 and 122.

In accordance with an important aspect of the invention, the resistorstring 120 is fabricated as polycrystalline silicon, with the headroomresistor 124 located adjacent thereto. To that end, the headroomresistor 124 provides the function of establishing a sufficient voltageoperating range for the current sources of the sub-resistor string 34,as well as functions to provide a dummy resistor for the main DACresistor string 120. As noted above, the utilization of the headroomresistor 124 adjacent the main DAC resistor string 120 compensates forthe street effect. Similarly, the other headroom resistor 126 functionsboth to provide the requisite voltage operating range for thesub-resistor string current sources, as well as dummy resistors for themain DAC companion resistor string 122. Because the headroom resistors124 and 126 function to provide two different features, a reduction inthe number of resistors is achieved, thereby reducing the semiconductorarea utilized, as well as the expense in fabrication of the device.

In order to further reduce the variations in the resistance values ofthe main DAC resistor strings 120 and 122, the top end 128 of theresistor string 120 is connected to circuit ground 130 via the offsetresistor 132. In contrast, the top 134 of the main DAC companionresistor string 122 is connected to the reference voltage Vref throughthe headroom resistor 126. Rather than cross connecting the nodes of theresistor strings 120 and 122 that are of similar voltages with metalconductors, such nodes are interconnected by way of transistor switches.Indeed, the transistor switches also serve dual purposes. First, thetransistor switches function to interconnect resistor string nodes ofsimilar voltages, as well as provide a decoding function for selecting aresistor of each main DAC resistor string for coupling the voltagethereof to the sub-resistor string 34.

A first switch multiplexer 136 includes a number of transistor switchesthat are connected across each resistor of the main DAC resistor string120. A second switch multiplexer 138 includes a similar number oftransistor switches connected across each resistor of the main DACcompanion resistor string 122. Various switches of the first switchmultiplexer 136 are connected to a first conductor 140 that is coupledto one end of the sub-resistor string 34. Other switches of the firstmultiplexer 136 are connected to another conductor 142 that is coupledto the other end of the sub-resistor string 34. The second switchmultiplexer 138 is similarly connected to select individual resistors ofthe main DAC companion resistor string 122. Some of the switches 138 areconnected to a third conductor 144, and the remainder of the switches ofthe second switch multiplexer 138 are connected to the common conductor146. Importantly, conductor 140 of the first switch multiplexer 136 isconnected to the conductor 144 of the second switch multiplexer 138. Inlike manner, the common conductor 142 of the first switch multiplexer136 is connected to the conductor 146 of the second switch multiplexer138.

The switches of the first multiplexer 136 and the second multiplexer 138are operated by a decoder (not shown) in such a manner that the switchesoperate together so that resistor 148 of the resistor string 120 isselected together at the same time with resistor 150 of companionresistor string 122. In other words, the decoder functions to closetransistor switch 152 together with transistor switch 154, as well asclose transistor switch 156 together with transistor switch 158. Byoperation of these switches, resistor string node 160 is short circuitedto resistor string node 162. Resistor string node 164 is connected viathe switches to resistor string node 166. If the two resistor stringnodes have different voltages, due to resistor process variations, thetwo voltages will be averaged and the result thereof output from theresistor string. By connecting together the resistor string nodes of theselected resistors, non-linear conversion errors are reduced. Inaddition, the switches 136 and 138 also serve to couple the voltageacross the parallel-connected resistors 148 and 150 to the sub-resistorstring 34 (FIG. 2). The other switches of the first multiplexer 136 aswell as the other switches of the second multiplexer 138 are effectiveto select each resistor of the 256 resistors to place the same inparallel, as well as to couple the voltage thereacross to thesub-resistor string 34. With this arrangement, each and every node ofthe resistor strings 120 and 122 can be connected together when selectedto reduce the non-linear errors and to couple the selected voltage tothe sub-resistor string 34. In practice, when the resistor strings 120and 122 are connected together through two transistor switches, it isnot necessary that the two switches have zero resistance. If theswitches have a finite resistance, the switches will still average orinterpolate the voltage between the two resistor nodes.

Not only is the headroom resistor 54 (FIG. 2) formed as two distinctresistors 124 and 126, but the offset resistor 56 (FIG. 2) is alsofabricated as separate polysilicon resistors 132 and 170. The respectiveends of the offset resistors 132 and 170 are connected to circuit ground130. Offset resistors 132 and 170 are connected in parallel as a resultof conductor 172 which short circuits together one end of each of theoffset resistors 132 and 170.

One hundred and ninety-two dummy resistors 174 are fabricated aspolysilicon resistors in line with the offset resistors 132 and 170. Thenumber of offset resistors 132 and 170, together with the dummyresistors 174, total 256 resistors in all. This combination of resistorsis physically located between the first main DAC resistor string 120 andthe companion main DAC resistor string 122. As such, the offsetresistors 132 and 170 and the dummy resistors 174 provide streetcompensation for the resistor strings 120 and 122. It should be notedthat the top end and the bottom end of the dummy resistor string 174 isshort circuited to ground. Thus, unlike the offset resistors 132 and 170which provide dual functions, the dummy resistors 174 provide the solefunction of street compensation for the main DAC resistor strings 120and 122.

FIG. 4 illustrates a layout showing the manner in which the main DACresistor strings 120 and 122 are cross connected by way of the firstswitch multiplexer 136 and the second switch multiplexer 138. Only aportion of the resistor strings 120 and 122 and the corresponding switchmultiplexer are shown. For purposes of clarity, the other functional anddummy resistors of the main DAC are not shown. As can be seen, theresistor string 120 is a continuous strip of polycrystalline silicon ofa desired resistivity. In practice, the resistor 120 is masked so as tobe serpentine-shaped, as is customary. Importantly, the polysiliconresistor strip 120 includes a number of lateral arms 180 for forming acontact 182 thereto. The contact 182 is not formed over or in the mainresistor strip 120. Thus, current flowing through the main resistorstrip 120 is not affected by any irregularity in the resistance that maybe caused by the contact 182. As noted above, a much smaller magnitudeof current, if any, flows through the contact 182, as compared to theresistor strip 120.

The contact 182 is connected to a conductor 184 that is connected to aPMOS transistor 186 and an NMOS transistor 188. The transistors 186 and188 define a first set of transistors comprising a transmission gate forselectively coupling the junction 180 of the resistor strip 120 to thecommon conductor 140. The second set of transistors comprising a PMOStransistor 190 and an NMOS 192 constitute a second transmission gate forcoupling the junction 180 of the resistor strip 120 to the other commonconductor 142. The gate terminals of the four transistors, one shown asreference numeral 194, are coupled to the decoding circuits (not shown)that control which resistor of the main DAC resistor string 120 is to beselected and the voltage thereacross coupled to the sub-resistor string34. The gates of the PMOS transistors and the NMOS transistors aredriven by complementary signals. Each of the other arms or junctions ofthe resistor string 120 is switched to a similar pair of transmissiongates to the respective common conductors 140 and 142. The second switchmultiplexer 138 includes similar pairs of transmission gates forcoupling the junctions of the companion resistor string 122 to thecommon conductors 144 and 146. The common conductor 140 of the firstswitch multiplexer 136 is connected to the common conductor 144 of thesecond switch multiplexer 138. In like manner, the common conductor 142of the first switch multiplexer 136 is connected to the common conductor146 of the second switch multiplexer 138. Hence, the first switchmultiplexer 136 and the second switch multiplexer 138 not only shortcircuit together nodes of the resistor strings 120 and 122 that are ofsimilar voltage, but also couple the voltage across the selectedresistors to the sub-resistor string 34.

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made therein without departing from the spirit and scope of theinvention, as defined by the appended claims.

What is claimed is:
 1. A digital-to-analog converter, comprising: afirst and second resistor string, each said resistor stringcharacterized by a plurality of resistors for providing plural analogvoltage levels; a differential amplifier having one input coupled to aprecision voltage, and another input coupled by a feedback circuit ofsaid amplifier to the first resistor string for driving said firstresistor string with a voltage reference; a current mirror circuit forcarrying current to said first resistor string, said current mirrorcircuit driven by said amplifier; a current source for driving saidsecond resistor string with a current, said current source including acurrent mirror driven by said amplifier, whereby changes in a currentdriven in said first resistor string are reflected in said secondresistor string; switching circuitry for selectively connecting one ofsaid plurality of resistors in said first resistor string across saidsecond resistor string; and an output amplifier for receiving an analogvoltage level resulting from said second resistor string.
 2. Thedigital-to-analog converter of claim 1, wherein said at least oneresistor string defines a sub-resistor string, and further including amain resistor string having a headroom resistor in series therewith,said headroom resistor providing an operating voltage for said currentsource.
 3. The digital-to-analog converter of claim 2, wherein said mainresistor string is driven by said current mirror circuit.
 4. Thedigital-to-analog converter of claim 3, wherein said headroom resistoris connected between said main resistor string and said current mirrorcircuit.
 5. The digital-to-analog converter of claim 2, wherein saidoutput amplifier is configured with feedback to provide a gain relatedto a headroom voltage developed across said headroom resistor.
 6. Thedigital-to-analog converter of claim 1, further including a transistorcurrent sink connected at one end of said second resistor string.
 7. Thedigital-to-analog converter of claim 6, wherein said transistor currentsink includes a current sink amplifier driving said transistor currentsink, and feedback between said transistor current sink and said currentsink amplifier to increase an output impedance to said transistorcurrent sink.
 8. The digital-to-analog converter of claim 7, whereinsaid current sink amplifier includes a current mirror functioning withsaid transistor current sink.
 9. The digital-to analog converter ofclaim 8, wherein said transistor current sink comprises a singletransistor in series with said second resistor string.
 10. Thedigital-to-analog converter of claim 9, further including an offsetresistor providing an operating voltage for said transistor currentsink.
 11. The digital-to-analog converter of claim 10, wherein saidoutput amplifier is configured with feedback to provide a gain relatedto an offset voltage developed across said offset resistor.
 12. Thedigital-to-analog converter of claim 1, wherein said plurality ofresistors of said first and second resistor strings are of substantiallythe same value.
 13. The digital-to-analog converter of claim 12, furtherincluding a supply voltage having a predetermined magnitude of voltage,and wherein said supply voltage is used to drive said current mirrorcircuit which supplies current to said first resistor string, and isused to drive said current mirror which supplies current to said secondresistor string.
 14. The digital-to-analog converter of claim 1, whereinsaid current source comprises a first current source for driving saidsecond resistor string with a current, and including a second currentsource operatively connected to drive a current sink connected to saidsecond resistor string.
 15. The digital-to-analog converter of claim 14,wherein said second current source includes a current mirror formirroring current to said current sink.
 16. The digital-to-analogconverter of claim 15, further including a third current source forestablishing a feedback between two transistors of said current mirrorthat mirrors current to said current sink.
 17. A digital-to-analogconverter, comprising: a main resistor string characterized by aplurality of resistors; a main current source for driving said mainresistor string with a current; a sub-resistor string characterized by aplurality of resistors; a sub-current source for driving saidsub-resistor string; a circuit for driving said main current source andsaid sub-current source with a signal such that a current in saidsub-resistor string tracks a current in said main resistor string;switching circuitry for selectively connecting one of said plurality ofresistors on said main resistor string across said sub-resistor string;and an output amplifier responsive to analog voltage levels resultingfrom said main and sub-resistor strings for producing correspondingoutput signals.
 18. The digital-to-analog converter of claim 17, furtherincluding a single transistor current sink connected in series with saidsub-resistor string.
 19. The digital-to-analog converter of claim 18,further including a first current mirror for mirroring current to saidsub-current source, and further including a second current mirror formirroring current to said current sink.
 20. The digital-to-analogconverter of claim 19, further including a feedback circuit coupledbetween said transistor current sink and said second current mirror. 21.A resistor voltage divider circuit, comprising: a main resistor stringcomprised of a plurality of resistors connected between first and secondnodes; a secondary resistor string comprised of a plurality of resistorsconnected between the third and fourth nodes; a voltage input forreceiving a reference voltage; circuitry for applying a voltage acrosssaid main resistor string substantially equal to said reference voltage;switching circuitry for selectively connecting said third and fourthnodes across one or more of said plurality of resistors in said mainresistor string, such that the voltage across said third and fourthnodes is substantially equal to the voltage across said one or more ofsaid plurality of resistors in said main resistor string selected bysaid switching circuitry; and a current source for sourcing of currentto said secondary resistor string.
 22. The resistor circuit of claim 21,wherein said current source is operable to drive said secondary resistorstring with a secondary current wherein the ratio of said secondarycurrent to said main current is substantially equal to the ratio of saidone or more of said plurality of resistors in said main resistor stringselected by said switching circuitry and the value of said plurality ofresistors in said secondary resistor string.
 23. The resistor circuit ofclaim 21 wherein said circuitry for applying a voltage across said mainresistor string substantially equal to said reference voltage comprisesa differential amplifier having one input coupled to said referencevoltage and another input coupled by a feedback circuit of saiddifferential amplifier to one side of said main resistor string and theoutput thereof driving a current mirror circuit that carries current tosaid first resistor string with a main current.
 24. The resistor circuitof claim 23, wherein said current source comprises a current mirror thatdrives said secondary resistor string with a secondary current that ismirrored from said current source driven by said differential amplifier.